Damn near anything, up to and including, a dead midget can be used as a tope.
I have ridden over ship ropes, steel tubes, concrete sewer pipes, you name it.
Never forget the reverse tope. This is where they hollow out the road, think concave and not convex.
Look at this video and you will see real Mexican topes and what happens when a guy who thinks he is Don Chingon goes for a ride. I think he showed up on the ground radar at Benito Juarez Airport.